This invention relates to machine tools, and in particular, to a system for detecting error in the positioning of the machine tool and for stopping machine tool operation before the mispositioning error causes destruction of the part under manufacture. While the invention is described with particular reference to its use in conjunction with the manufacturing of aircraft frame parts, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
Computer controlled machine tools have become generally available for the manufacture of precision, close tolerance parts. One generally available machine tool includes a machine bed for holding the parts and a gantry having a suitable milling device mounted to it. The gantry rides above the bed and both the gantry and milling device are controlled by a general purpose computer. The milling device commonly is movable in three directions of movement, those being the three planes of a conventional Cartesian Coordinate system denominated as the X, Y and Z planes, along with clockwise and counterclockwise rotational movement in each of those planes. Suitable drive motors are provided for driving the milling device along the X, Y and Z axes of the coordinate system. The direction of rotation and the position of the milling device along the X, Y and Z axes commonly are a function of a computer input to the machine tool. Other conventional machine tools provide at least a work surface for holding a workpiece and means for moving the cutter with respect to the workpiece.
Machine tools as thus generally described are well known in the art. They are used extensively by aircraft manufacturers, for example, to construct precision frame members used in the air frames of general purpose and military aircraft. Because of their precise construction, individual parts manufactured on the machine tools which the invention disclosed hereinafter finds application represent a substantial investment. Machine down time also represents a serious loss in productivity with resulting increased product cost.
Most computer operated machine tool devices provide some form of servo error detection on an individual axis basis. Commonly utilized servo error detection circuitry utilized in conjunction with those devices use the electronic measurement of following error to determine machine malfunction. A malfunction is detected only when the following error is very large, generally on the order of 0.100 to 0.200 inches, or larger. The resolution is very coarse since the normal following error of the machine tool should not cause servo error indication. An improved monitoring system has been developed which checks the magnitude of the machine following error against a predicted value based on velocity of the slide holding the particular milling tool. While the sensitivity of this latter device is an order of magnitude better, i.e., it may detect positioning error of 0.010 inches, the system only checks the performance of the servo electronics, and does not take into account errors in the input command decoding logic. Additionally, both of the systems described above depend on the tool's existing feedback system to perform their checks. Consequently, a number of malfunctions still may occur with the resultant loss of machine time and material.
The invention described hereinafter overcomes these prior art deficiencies by implementing a system employing a total redundancy check concept. That is to say, the system monitors the commands to the machine tool in parallel with a machine control unit conventionally associated with the machine tool, and independently decodes them. Machine tool position is monitored with independent feedback devices on each axis. The system compares machine tool position with the mathematically ideal path commanded by the controlling computer. Thus, both individual single axis error and vector error caused by several small axis errors can be detected. In effect, a monitoring loop is put around the entire machine tool system, the loop being capable of detecting any malfunction concerning the position of the tool. The system also monitors the status of the machine control unit and requires certain functions to be performed by the machine operator before the operator can place the machine in automatic control operation. This monitoring function eliminates many areas of possible human error.
One of the objects of this invention is to provide an improved system for ensuring the reliablity of machine tool operation.
Another object of this invention is to provide a reliability system for a machine tool which monitors the entire machine tool operation.
Another object of this invention is to provide a monitoring system which compares the mathematically ideal position of a machine tool with the position of the tool as determined by sensors which function independently of machine tool operation.
Another object of this invention is to provide a dynamic braking system for shutting down machine tool operation upon the detection of a position error of the tool.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.